Electroluminescent fluorinated pyrenes, and devices made with such compounds

ABSTRACT

This invention relates to electroluminescent fluorinated pyrene compounds. It also relates to electronic devices in which the active layer includes an electroluminescent fluorinated pyrene compound.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electroluminescent fluorinated pyrene compounds. It also relates to electronic devices in which the active layer includes an electroluminescent fluorinated pyrene compound.

2. Description of the Related Art

Organic electronic devices that emit light, such as light-emitting diodes (LEDs) that make up displays, are present in many different kinds of electronic equipment. In all such devices, an organic active layer is sandwiched between two electrical contact layers. At least one of the electrical contact layers is light-transmitting so that light can pass through the electrical contact layer. The organic active layer emits light through the light-transmitting electrical contact layer upon application of electricity across the electrical contact layers.

It is well known to use organic electroluminescent compounds as the active component in light-emitting diodes. Simple organic molecules such as anthracene, thiadiazole derivatives, and coumarin derivatives are known to show electroluminescence.

JP2001/111868 discloses 1,3,6,8-tetraphenylpyrene, alkyl and cycloalkyl derivatives of 1,3,6,8-tetraphenylpyrene, and the use of such fluorescent compounds in electronic light emitting devices.

However, there is a continuing need for electroluminescent compounds.

SUMMARY OF THE INVENTION

The present invention is directed to compounds represented by Formula I:

wherein:

-   -   R¹—R⁴═H or comprises an F-substituted or fluoroalkyl-substituted         aryl group;     -   wherein at least one of R¹—R⁴ comprises an F-substituted or         fluoroalkyl-substituted aryl group.

In another embodiment, the present invention is directed to an organic electronic device having at least one photoactive layer positioned between two electrical contact layers, wherein the at least one photoactive layer comprises a compound represented by Formula I.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a light-emitting device (LED).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The compounds of the invention are represented by Formula I:

wherein:

-   -   R¹—R⁴═H or comprises an F-substituted or fluoroalkyl-substituted         aryl group; and     -   wherein at least one of R¹—R⁴ comprises an F-substituted or         fluoroalkyl-substituted aryl group.

Each of the substituents, R¹—R⁴, can be selected independently, but for ease of synthesis it is preferred that R¹═R²═R³═R⁴, i.e., that each R¹—R⁴ comprise the same F-substituted or fluoroalkyl-substituted aryl group.

The F-substituted or fluoroalkyl-substituted aryl groups of R¹—R⁴ can be additionally and independently substituted with C₁-C₁₀ alkyl, C₁-C₁₀ haloalkyl, C₆-C₁₀ aryl, C₁-C₁₀ alkoxy, C₁-C₁₀ haloalkoxy, halo, or formyl groups. For example, each of R¹—R⁴ can be selected from the following:

Each of R¹—R⁴ can contain more than one fluoro- or fluoroalkyl group.

The compounds represented by Formula I are readily synthesized via the so-called Suzuki coupling reaction. A Pd-catalyzed reaction of the appropriate mono-, di-, tri- or tetrachloro-substituted pyrenes with the appropriate fluorine-substituted or fluoroalkyl-substituted arylboronic acid gives the desired compounds represented by Formula I. In general, a slight excess of the boronic acid is used, relative to the number of chlorines on the pyrene. A base, e.g., cesium carbonate, is used to neutralize the HCl that is liberated in the coupling reaction. Tris(dibenzylideneacetone)dipalladium is a convenient catalyst to use, since its ligands are labile, but do not react with the other reagents under the reaction conditions. Other Pd catalysts can also be used. The phosphanes serve as stabilizing ligands for the Pd catalyst.

In one embodiment, the compounds represented by Formula I exhibit blue luminescence. In one embodiment, the compounds have photoluminescent and/or electroluminescent spectra that have a maximum at 500 nm or less.

The pyrene compounds of this invention can be easily sublimed or volatilized. Thin films of these materials obtained via vacuum deposition exhibit good to excellent electroluminescent properties. It has been found that introduction of fluorine-containing substituents onto the pyrene ring increases the volatility of the pyrenes and also makes them more readily soluble in organic solvents. Such increases in volatility and solubility provide increased processability of the compounds of this invention.

Examples of fluorine-substituted pyrene compounds of this invention include, but are not limited to, compounds represented by Formulas II and III, shown below.

Electronic Device

The present invention also relates to an electronic device comprising at least one photoactive layer positioned between two electrical contact layers, wherein the at least one layer of the device includes the fluorine-substituted compound of the invention. Devices frequently have additional hole transport and electron transport layers. A typical structure is shown in FIG. 1. The device 100 has an anode layer 110 and a cathode layer 150. Adjacent to the anode is a layer 120 comprising hole transport material. Adjacent to the cathode is a layer 140 comprising an electron transport material. Between the hole transport layer and the electron transport layer is the photoactive layer 130.

Depending upon the application of the device 100, the photoactive layer 130 can be a light-emitting layer that is activated by an applied voltage (such as in a light-emitting diode or light-emitting electrochemical cell), a layer of material that responds to radiant energy and generates a signal with or without an applied bias voltage (such as in a photodetector). Examples of photodetectors include photoconductive cells, photoresistors, photoswitches, phototransistors, and phototubes, and photovoltaic cells, as these terms are describe in Markus, John, Electronics and Nucleonics Dictionary, 470 and 476 (McGraw-Hill, Inc. 1966).

The fluorine-substituted compounds of the invention are particularly useful as the photoactive material in layer 130, or as electron transport material in layer 140. Preferably the fluorine-substituted compounds of the invention are used as the light-emitting material in diodes. Additional materials can be present in the emitting layer with the fluorine-substituted pyrene compound. For example, a fluorescent dye may be present to alter the color of emission. A diluent may also be added and such diluent may be a charge transport material or an inert matrix. A diluent may comprise polymeric materials, small molecule or mixtures thereof. A diluent may act as a processing aid, may improve the physical or electrical properties of films containing the fluorine-substituted pyrene compound, may decrease self-quenching in the pyrene compounds described herein, and/or may decrease the aggregation of the pyrene compounds described herein. Non-limiting examples of suitable polymeric materials include poly(N-vinyl carbazole) and polysilane. Non-limiting examples of suitable small molecules includes 4,4′-N,N′-dicarbazole biphenyl or tertiary aromatic amines. When a diluent is used, the fluorine-substituted pyrene compound is generally present in a small amount. In one embodiment, the pyrene compound is less than 20% by weight, based on the total weight of the layer. In one embodiment, the pyrene compound is less than 10% by weight, based on the total weight of the layer.

To achieve a high efficiency LED, the HOMO (highest occupied molecular orbital) of the hole transport material should align with the work function of the anode, the LUMO (lowest un-occupied molecular orbital) of the electron transport material should align with the work function of the cathode. Chemical compatibility and sublimation temperature of the materials are also important considerations in selecting the electron and hole transport materials.

The other layers in the OLED can be made of any materials that are known to be useful in such layers. The anode 110, is an electrode that is particularly efficient for injecting positive charge carriers. It can be made of, for example materials containing a metal, mixed metal, alloy, metal oxide or mixed-metal oxide, or it can be a conducting polymer. Suitable metals include the Group 11 metals, the metals in Groups 4, 5, and 6, and the Group 8-10 transition metals. If the anode is to be light-transmitting, mixed-metal oxides of Groups 12, 13 and 14 metals, such as indium-tin-oxide, are generally used. The IUPAC numbering system is used throughout, where the groups from the Periodic Table are numbered from left to right as 1-18 (CRC Handbook of Chemistry and Physics, 81^(st) Edition, 2000). The anode 110 may also comprise an organic material such as polyaniline as described in “Flexible light-emitting diodes made from soluble conducting polymer,” Nature vol. 357, pp 477479 (11 Jun. 1992). At least one of the anode and cathode should be at least partially transparent to allow the generated light to be observed.

Examples of hole transport materials for layer 120 have been summarized for example, in Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Vol.18, p. 837-860, 1996, by Y. Wang. Both hole transporting molecules and polymers can be used. Commonly used hole transporting molecules are: N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]4,4′-diamine (TPD), 1,1-bis[(di-4-tolylamino) phenyl]cyclohexane (TAPC), N,N′-bis(4-methylphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]4,4′-diamine (ETPD), tetrakis-(3-methylphenyl)-N,N,N′,N′-2,5-phenylenediamine (PDA), a-phenyl-4-N,N-diphenylaminostyrene (TPS), p-(diethylamino)benzaldehyde diphenylhydrazone (DEH), triphenylamine (TPA), bis[4-(N,N-diethylamino)-2-methylphenyl](4-methylphenyl)methane (MPMP),1-phenyl-3-[p-(diethylamino)styryl]-5-[p-(diethylamino)phenyl] pyrazoline (PPR or DEASP), 1,2-trans-bis(9H-carbazol-9-yl)cyclobutane (DCZB), N,N,N′,N′-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TTB), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB or NPD), carbazole biphenyl (CBP), and porphyrinic compounds, such as copper phthalocyanine. Commonly used hole transporting polymers are polyvinylcarbazole, (phenylmethyl)polysilane, polypyrroles, polythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDOT), and polyaniline. It is also possible to obtain hole transporting polymers by doping hole transporting molecules such as those mentioned above into polymers such as polystyrene and polycarbonate.

Examples of electron transport materials for layer 140 include metal chelated oxinoid compounds, such as tris(8-hydroxyquinolato)aluminum (Alq₃); phenanthroline-based compounds, such as 2,9-dimethyl4,7-diphenyl-1,10-phenanthroline (DDPA) or 4,7-diphenyl-1,10-phenanthroline (DPA), and azole compounds such as 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD) and 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (TAZ). Layer 140 can function both to facilitate electron transport, and also serve as a buffer layer or confinement layer to prevent quenching of the exciton at layer interfaces. Preferably, this layer promotes electron mobility and reduces exciton quenching.

The cathode 150, is an electrode that is particularly efficient for injecting electrons or negative charge carriers. The cathode can be any metal or nonmetal having a lower work function than the anode. Materials for the cathode can be selected from alkali metals of Group 1 (e.g., Li, Cs), the Group 2 (alkaline earth) metals, the Group 12 metals, including the rare earth elements and lanthanides, and the actinides. Materials such as aluminum, indium, calcium, barium, samarium and magnesium, as well as combinations, can be used. Li-containing organometallic compounds can also be deposited between the organic layer and the cathode layer to lower the operating voltage.

It is known to have other layers in organic electronic devices. For example, there can be a layer (not shown) between the conductive polymer layer 120 and the active layer 130 to facilitate positive charge transport and/or band-gap matching of the layers, or to function as a protective layer. Similarly, there can be additional layers (not shown) between the active layer 130 and the cathode layer 150 to facilitate negative charge transport and/or band-gap matching between the layers, or to function as a protective layer. Layers that are known in the art can be used. In addition, any of the above-described layers can be made of two or more layers. Alternatively, some or all of inorganic anode layer 110, the conductive polymer layer 120, the active layer 130, and cathode layer 150, may be surface treated to increase charge carrier transport efficiency. The choice of materials for each of the component layers is preferably determined by balancing the goals of providing a device with high device efficiency.

It is understood that each functional layer may be made up of more than one layer.

The device can be prepared by sequentially vapor depositing the individual layers on a suitable substrate. Substrates such as glass and polymeric films can be used. Conventional vapor deposition techniques can be used, such as thermal evaporation, chemical vapor deposition, and the like. Alternatively, the organic layers can be coated from solutions or dispersions in suitable solvents, using any conventional coating technique. In general, the different layers will have the following range of thicknesses: anode 110, 500-5000 Å, preferably 1000-2000 Å; hole transport layer 120, 50-1000 Å, preferably 200-800 Å; light-emitting layer 130, 10-1000 Å, preferably 100-800 Å; electron transport layer 140, 50-1000 Å, preferably 200-800 Å; cathode 150, 200-10000 Å, preferably 300-5000 Å. The location of the electron-hole recombination zone in the device, and thus the emission spectrum of the device, can be affected by the relative thickness of each layer. Thus the thickness of the electron-transport layer should be chosen so that the electron-hole recombination zone is in the light-emitting layer. The desired ratio of layer thicknesses will depend on the exact nature of the materials used.

It is understood that the efficiency of devices made with the fluorine-substituted pyrene compounds of the invention, can be further improved by optimizing the other layers in the device. For example, more efficient cathodes such as Ca, Ba or LiF can be used. Shaped substrates and novel hole transport materials that result in a reduction in operating voltage or increase quantum efficiency are also applicable. Additional layers can also be added to tailor the energy levels of the various layers and facilitate The fluorine-substituted pyrene compounds of the invention often are photoluminescent and may be useful in applications other than OLEDs.

The phrase “adjacent to,” when used to refer to layers in a device, does not necessarily mean that one layer is immediately next to another layer. On the other hand, the phrase “adjacent R groups,” is used to refer to R groups that are next to each other in a chemical formula (i.e., R groups that are on atoms joined by a bond). The term “photoactive” refers to any material that exhibits electroluminescence and/or photosensitivity.

As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

Also, use of the “a” or “an” are employed to describe elements and components of the invention. This is done merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

EXAMPLES

The following examples illustrate certain features and advantages of the present invention. They are intended to be illustrative of the invention, but not limiting. All percentages are by weight, unless otherwise indicated.

All reagents used in the following examples were purchased from Sigma-Aldrich Corporation, Milwaukee, Wis., except for di-tert-butyl-trimethylsilylmethyl-phosphane and MPMP.

Preparation of Di-tert-butyl-trimethylsilylmethyl-phosphane ^(t)Bu₂P—CH₂—SiMe₃

50.00 g (0.277 mol) of di-t-butylchlorophosphine, 304 ml of 1.0 M pentane solution of (trimethylsilylmethyl)lithium and 150 ml of THF) were refluxed under argon for 3 days. The reaction mixture was allowed to cool to RT and an aqueous solution of ammonium chloride was added slowly. The organic phase was separated, and dried with magnesium sulfate. After removal of the solvent, the product was purified by distillation under vacuum. The yield of di-tert-butyl-trimethylsilylmethyl-phosphane was 55.32 g (86%) with b.p. 50-52° C./0.5 mm. 31-P-NMR (C6D6)+20.05 ppm. 1H NMR (C6D6) 0.01 (s, 9H, SiMe3 ),0.23 (d, 2H, 2JPH=5.34Hz, P—CH2—SiMe3), 0.91 (s, 9H, Me3C), 0.93 (s, 9H, Me3C). Elemental analysis: C, 61.89; H, 12.53; P, 13.25.

MPMP was prepared in a manner similar to that for bis(4-diethylamino-2-methylphenyl)phenylmethane, as described in U.S. Pat. No. 4,053,311 (Example 1), except that p-tolualdehyde was substituted for benzaldehyde.

Example 1 1,3,6,8-Tetrakis-(3,5-bis-trifluoromethyl-phenyl)-pyrene

3.95 g (0.0116 mol) of 1,3,6,8-tetrachloro-pyrene, 15.0 g (0.0582 mol) of 3,5-bis(trifluoromethyl)phenylboronic acid, 1.33 g (0.00145 mol) of tris(dibenzylideneacetone) dipalladium (0), 0.64 g (0.00276 mol) of di-tert-butyl-trimethylsilylmethyl-phosphane, 18.95 g (0.0582 mol) of cesium carbonate and 100 ml of dioxane were stirred at room temperature for 24 hours. The resultant mixture was poured into 200 ml of water and extracted twice with 200 ml of methylene chloride. The organic phase was dried over magnesium sulfate overnight and filtered. The solvent was removed on a roto-evaporator and the residue was purified by chromatography on silica gel with petroleum ether/ethyl ether (10/0.5) as eluent. Yield of 1,3,6,8-tetrakis-(3,5-bis-trifluoromethyl-phenyl)-pyrene: 1.15 g (9.42%) as a white solid with m.p. 313.85° C. ¹H NMR (CD₂Cl₂) 7.90-8.30 (multiplets, 18H, arom-H). ¹⁹F NMR (CDCl₃) −63.70. The structure was confirmed by X-ray analysis.

Excitation and emission spectra were acquired on both the powder and a dichloromethane solution of 1,3,6,8-tetrakis-(3,5-bis-trifluoromethyl-phenyl)-pyrene. The emission of the solution [max˜410 nm] is significantly blue-shifted from that of the powder [max˜450 nm]. The quantum yield is at least 0.6 [versus quinine sulfate]. There was a second, lower intensity emission at ˜600 nm from the powder, which was not seen in very dilute solution, used for the quantum yield.

Example 2 1,3,6,8-Tetrakis-(2,4-bis-trifluoromethyl-phenyl)-pyrene

3.95 g (0.0116 mol) of 1,3,6,8-tetrachloro-pyrene, 15.0 g (0.0582 mol) of 2,4-bis(trifluoromethyl)phenylboronic acid, 1.33 g (0.00145 mol) of tris(dibenzylideneacetone) dipalladium (0), 0.64 g (0.00276 mol) of di-tert-butyl-trimethylsilylmethyl-phosphane, 18.95 g (0.0582 mol) of cesium carbonate and 100 ml of dioxane were stirred at room temperature for 24 hours. The resultant mixture was poured into 200 ml of water and extracted twice with 200 ml of methylene chloride. The organic phase was dried over magnesium sulfate overnight and filtered. The solvent was removed on a roto-evaporator and the residue was purified by chromatography on silica gel with petroleum ether/ethyl ether (10/0.5) as eluent. Yield of 1,3,6,8-tetrakis-(2,4-bis-trifluoromethyl-phenyl)-pyrene: 1.08 g (8.85%) as a white solid with m.p. 290.00° C. ¹H NMR (CDCl₃) 7.45-8.30 (multiplets, 18H, arom-H). ¹⁹F NMR (CDCl₃) −59.16, −63.27. The structure was confirmed by X-ray analysis.

Example 3 1,3,6,8-Tetrakis-pentafluorophenyl-pyrene

4.81 g (0.0141 mol) of 1,3,6,8-tetrachloro-pyrene, 15.0 g (0.0708 mol) of pentafluorophenylboronic acid, 1.62 g (0.00177 mol) of tris(dibenzylideneacetone) dipalladium (0), 0.99 g (0.00427 mol) of di-tert-butyl-trimethylsilylmethyl-phosphane, 23.06 g (0.0708 mol) of cesium carbonate and 100 ml of dioxane were stirred at room temperature for 24 hours. The resultant mixture was poured into 200 ml of water and extracted twice with 200 ml of methylene chloride. The organic phase was dried over magnesium sulfate overnight and filtered. The solvent was removed on a roto-evaporator and the residue was purified by chromatography on silica gel with petroleum ether/ethyl ether (10/0.5) as eluent. Yield of 1,3,6,8-tetrakis-pentafluorophenyl-pyrene: 0.78 g (6.37%) as a white solid with m.p. 305.45° C. ¹H NMR (CDCl₃) 7.90-8.60 (multiplets, 6H, arom-H). ¹⁹F NMR (CDCl₃)−141.5 (broad, 8F), −156.80 (broad, 4F), −164.40 (broad, 8F). GC/MS for C₄₀H₆F₂₀.

Example 4

OLED devices were fabricated by the thermal evaporation technique. The base vacuum for all of the thin film deposition was in the range of 10⁻⁶ torr. The deposition chamber was capable of depositing eight different films without the need to break up the vacuum.

Patterned indium tin oxide (ITO) coated glass substrates from Thin Film Devices, Inc were used. These ITO's are based on Corning 1737 glass coated with 1400 Å ITO coating, with sheet resistance of 30 ohms/square and 80% light transmission. The patterned ITO substrates were then cleaned ultrasonically in aqueous detergent solution. The substrates were then rinsed with distilled water, followed by isopropanol, and then degreased in toluene vapor for ˜3 hours.

The cleaned, patterned ITO substrate was then loaded into the vacuum chamber and the chamber was pumped down to 10⁻⁶ torr. The substrate was then further cleaned using an oxygen plasma for about 5 minutes. After cleaning, multiple layers of thin films were then deposited sequentially onto the substrate by thermal evaporation. Patterned metal electrodes (Al or LiF/Al) or bipolar electrode were deposited through a mask. The thickness of the film was measured during deposition using a quartz crystal monitor (Sycon STC-200). All film thickness reported in the Examples are nominal, calculated assuming the density of the material deposited to be one. The completed OLED device was then taken out of the vacuum chamber and characterized immediately without encapsulation.

The OLED samples were characterized by measuring their (1) current-voltage (I-V) curves, (2) electroluminescence radiance versus voltage, and (3) electroluminescence spectra versus voltage. The I-V curves were measured with a Keithley Source-Measurement Unit Model 237. The electroluminescence radiance (in the unit of cd/m²) vs. voltage was measured with a Minolta LS-110 luminescence meter, while the voltage was scanned using the Keithley SMU. The electroluminescence spectrum was obtained by collecting light using an optical fiber, through an electronic shutter, dispersed through a spectrograph, and then measured with a diode array detector. All three measurements were performed at the same time and controlled by a computer. The efficiency of the device at certain voltage is determined by dividing the electroluminescence radiance of the LED by the current density needed to run the device. The unit is in cd/A.

Table I summarizes device configuration and efficiency of OLED devices fabricated using materials disclosed in the present invention. MPMP is the hole transport material, DPA is the electron transport material, AlQ is the electron injection material, and CBP (carbazole biphenyl) is the host material. Their molecular structures are:

TABLE I Device configurations and efficiency of OLED device Peak wave- Device Efficiency, Radiance, length, Color Emitter configuration cd/A cd/m2 nm coordinates

MPMP (303 Å)/emitter 1 (401 Å)/DPA (104 Å)/AlQ (305 Å)/LiF (10 Å)/Al (503 Å) 0.45 at 15 V 300 at 22 V ~580

MPMP (304 Å)/emitter 1 (51 Å)/CBP (524 Å)/DPA (104 Å)/AlQ (304 Å)/LiF (10 Å)/Al (504 Å) 0.6 at 13 V 150 at 23 V 445 (0.21, 0.348) 

1. A compound represented by Formula I

wherein: each of R¹-R⁴ ═H or an F-substituted aryl group or fluoroalkyl-substituted aryl group or a fluoroalkoxy-substituted aryl group; and wherein at least one of R¹-R⁴ is: an F-substituted aryl group selected from the group consisting of 2,3,4-trifluorophenyl; 2,3,6-trifluorophenyl; 2,4,6-trifluorophenyl; 3,4,5-trifluorophenyl; 2-chloro-6-fluorophenyl; 2-chloro-6-fluoro-3-methylphenyl; 2-chloro-6-fluoro-5-methylphenyl; 4-fluoro-3-methylphenyl; 2-fluoro-6-methoxyphenyl; 3-fluoro-4-methoxyphenyl; 3-fluoro-4-methoxyphenyl; 2-fluoro-4-biphenyl; trans-2-(fluorophenyl)vinyl; or a fluoroalkyl-substituted aryl group selected from the group consisting of trans-2-(4-trifluoromethylphenyl)vinyl; 3,5-bis(trifluoromethyl)phenyl; and 2,4-bis(trifluoromethyl)phenyl; or a fluoroalkoxy-substituted aryl group selected from the group consisting of 3-(trifluoromethoxy)phenyl and 4-(trifluoromethoxy)phenyl.
 2. A compound represented by Formula II or Formula III: 